The only issue with mounting the electric motor in front of the axle with no driveshaft is it adds a substantial amount of unsprung weight to the rear suspension system. While not an insurmountable issue, it will require thought and re-engineering to compensate for. Making clearance for the motor in this setup might also require giving up some interior space. Perhaps flipping the axle so that the electric motor was behind the axle would be an option to save usable interior space. This would mandate a custom axle of course.
As for least intrusive packaging I think a motor hung in the transmission tunnel about 12-14 inches ahead of the rear axle with a CV drive shaft would be simplest. But clearance for speed bumps and rod debris may be an issue.
But in reality converting an ICE car into an EV is going to cost an exorbitant amount of money. The ROI will likely NEVER work out. The car will probably wear out before you break even. Or worse get totaled resulting in lost money for the conversion parts.
Look at the $9,000 for just batteries. I think it'll cost at least 3X that before your done. Today gas is about $3.70/gallon in San Diego (per gasbuddy.com). $27,000 will buy 7,297ish gallons. If your car gets only 20 mpg (totally doable if fuel economy is the goal), you can drive over 145,900 miles. How many miles a year do you drive? Average miles driven by Californians is about 14,400 per year. That's a little over 10 years just to break even.
I don't think the economics are there. You would be further ahead to buy something already made to be an EV. I bet there are used Tesla's or other EV's in the $30K price range at this point.
Your points are excellent and are exactly the reasons I flip back and forth between "coyote swap" and "electric conversion".
Please consider the following though:
1. Mustangs are NEVER about ROI... Mustangs are exercises in converting money into frustration.
2. An axle can't be operated upside down as the fluid would leak out the vents. But spinning it about the vertical axis, and presuming one can somehow attach a torque arm and LCAs, if the motor were attached to the axle and mounted (somehow) where the fuel tank presently is, everything should work just fine. Although as you stated, the unsprung weight is going to need addressing.... that much independent mass needs some serious study, and let's just agree right now that it's impractical.
3. Weight redistribution. A 32V Mustang has something like 60% of its weight over the front wheels, and much of that weight is way up high, about two thirds the way up the engine. Removing that engine and putting an electric motor in the transmission tunnel, along with batteries int he fuel tank location, significantly lowers the car's center of gravity while shifting the weight distribution back to the rear wheels. Such a car would corner like madness.
Plus, for those not familiar with modern electric cars, acceleration off the line is a strong-point, and doubly so with traction over the drive wheels.
4. Great point about speed bumps hitting the motor. I believe the motor would fit, but worse case I should consider a strike plate.
5. Cost. A proper coyote swap is $20k. MM front end, driver side cat, gen3 package (T56 et all) , sn95 accessory bracket and driveshaft. A proper E conversion is about that. The gas motor will involve the hassle of weekly visits to the gas station, biannual smog checks, oil changes every 5000 miles, and the frustration of idling away gasoline while sitting at a red light or in traffic, but will be a more comprehensive and finished package. And downright cool. The cruise control and "all the little things" should work, and the car should be good for cross-country trips.
An electric conversion will involve lots of customization and frankly I'm not sure how to even get a working speedo. But electric cars with lithium batteries (versus golf carts of yore) are sooo much funnnnnnnnn. I have an e-golf, and the little things like bad traffic just don't matter any more, since I'm serenely listening to the radio (versus a thunderous idle) and every morning I have a fully charged car, so no trips to the gas station anymore. And when people freak about increased gas prices I breath a sigh of relief that it doesn't affect me.
Plus, did you see that the dual motor has 1000 ft lbs of torque?